As a display device for OA devices such as word processor, note personal computer and monitor for personal computer, terminals of cellular phone, television, etc., there has heretofore been mainly used CRT (cathode ray tube). In recent years, liquid crystal displays have been widely used instead of CRT due to their small thickness, light weight and small power consumption.
A liquid crystal display comprises a liquid crystal cell and a polarizing plate. The polarizing plate comprises a protective film and a polarizing film. The polarizing film is obtained by dyeing a polarizing film made of polyvinyl alcohol film with iodine, stretching the polarizing film thus dyed, and then superposing a protective film on the both sides of the polarizing film. A transmissive liquid crystal display comprises this polarizing plate mounted on the both sides of a liquid crystal cell and optionally one or more optical compensation films provided thereon. A reflective liquid crystal display comprises a reflective plate, a liquid crystal cell, one or more optical compensation films and a polarizing plate provided in this order. The liquid crystal cell comprises liquid crystal molecules, two sheets of substrates for enclosing the liquid crystal molecules therein and an electrode layer for applying a voltage to the liquid crystal molecules. As liquid crystal cells there have been proposed those of display modes such as TN (Twisted Nematic), IPS (In-Plane Switching), OCB (Optically Compensatory Bend), VA (Vertically Aligned), ECB (Electrically Controlled Birefringence) and STN (Super Twisted Nematic), which perform ON or OFF display depending on the alignment of the liquid crystal molecules and can be applied to all the transmissive, reflective and semi-transmissive liquid crystal displays.
The optical compensation film is used in various liquid crystal displays to eliminate image discoloration or enhance the viewing angle. As the optical compensation film there has heretofore been used a stretched birefringent polymer film. It has been proposed to use an optical compensation film having an optical anisotropic layer made of a low or high molecular liquid crystalline compound provided on a transparent support instead of the optical compensation film made of stretched birefringent film. The liquid crystalline compound is aligned in various configurations. Thus, the use of the liquid crystalline compound makes it possible to realize optical properties which cannot be obtained with the related art stretched birefringent polymer film. The liquid crystalline compound acts also as a protective film for polarizing plate.
The optical properties of the optical compensation film are determined by the optical properties of the liquid crystal cell, i.e., aforementioned difference of display mode. The use of the liquid crystalline compound makes it possible to produce an optical compensation film having various optical properties corresponding to the various display modes of liquid crystal cell. An optical compensation film comprising a liquid crystalline compound which can operate in various display modes has been already proposed. For example, when a voltage is applied, an optical compensation film for TN mode liquid crystal cell performs optical compensation such that the liquid crystal molecules are aligned obliquely to the surface of the substrate while eliminating the twisted structure thereof, making it possible to enhance the front contrast ratio of the display device and prevent the leakage of light in oblique direction during black display and hence improve the viewing-angle properties of contrast.
Representative examples of optical compensation methods include a method involving the disposition of a set of stretched films having the same phase difference stacked crossing each other to attain an in-plane retardation close to zero between upper and lower polarizing plates and a liquid crystal cell (see JP-A-4-162018).
However, when a voltage is applied, the molecules in the liquid crystal cell are not completely aligned perpendicular to the substrate and those disposed in the periphery of the substrate are left aligned parallel to the substrate. On the other hand, the liquid crystal molecules disposed in the middle region of the substrate are aligned perpendicular to the substrate and the liquid crystal molecules ranging from the middle region of the substrate to the region close to the periphery of the substrate are continuously aligned obliquely to the substrate. In order to optically compensate the aforementioned alignment of the liquid crystal cell, the optical compensation film is preferably provided with the same optical properties.
As a technique for providing such an optical compensation film there has been proposed a technique which comprises forming a liquid crystalline compound into a film while being hybrid-aligned. As such a liquid crystalline compound there is used a discotic liquid crystalline compound (see JP-A-6-214116) or a rod-shaped liquid crystalline compound (see JP-A-10-186356).
However, even when an optical compensation film obtained by uniformly hybrid-aligning a discotic liquid crystalline compound is used, it is extremely difficult to make complete optical compensation of a liquid crystal cell without any problems. For example, TN mode liquid crystal cells show brightness inversion, which is transmittance inversion at various gradations, as viewed obliquely. As an approach for preventing the occurrence of brightness inversion there is known a method involving the limitation of the tilt angle range of the liquid crystal molecules in the liquid crystal cell (see TECHNICAL REPORT OF IEICE, EID2001-108, pp. 47-52). However, this method leaves something to be desired.